Final answer:
After delivering a shock or when an AED indicates no shock advised, the immediate step is to begin chest compressions to maintain circulation and follow the AED's voice instructions.
Step-by-step explanation:
When using an Automated External Defibrillator (AED), after a shock has been delivered or when the AED prompts 'no shock advised,' the next immediate action should be to begin chest compressions. This step is crucial because it maintains circulation to vital organs, particularly the brain and heart. Continue following the unit's voice instructions which often guide you through the CPR process. It's imperative to start or resume CPR quickly to maximize the chances of survival.